ES2289356T3 - THERMAL CONFORMATION OF TWIN SHEETS OF FUEL PLASTIC DEPOSITS. - Google Patents

Abstract

A method and a plant for twin-sheet thermoforming of fuel tanks; first and second sheets (SA, SB) of thermoformable plastic material, are separately fed along respective processing lines (10A, 10B). The sheets (SA, SB) are heated and gripped along their peripheral edges by a pneumatically actuate suction and vacuum holding device (15A, 15B), for supporting the heated sheets (SA, SB) in a substantially flat condition while they are moved towards a respective thermoforming station (16A, 16B). Both the molds (17A, 17B) are disposed side by side with their open cavity facing upwards. After thermoforming of the plastic sheets (SA, SB), one of the molds (17B) is turned upside down onto the other mold (17A), to overlap and weld superimposed sealing areas of the two thermoformed shells (GA, GB). Cooling of the molds and thermoformed tank may be performed on a side of the processing lines (10A, 10B).

Description

Thermal conformation of twin leaves of plastic fuel tanks.

Field of the Invention

This invention relates to the conformation thermal plastic fuel tanks that have about high structural characteristics and a gas barrier, starting from layered sheets of a plastic material, such as high density polyolefins with an EVOH barrier for containment of fuel vapors; in particular the invention relates to a method for manufacturing deposits of fuel by thermal forming technology of "twin leaves", and to the corresponding plant.

Prior art

Metal fuel tanks are used widely in various fields, for example for the supply of fuel to internal combustion engines, both in vehicles terrestrial as nautical and on airplanes, as well as for others applications; however, metal fuel tanks currently in use they are heavy, difficult to shape and are subjected to corrosion.

The current trend is to replace the metal fuel tanks for fuel tanks of Layered plastic material, due to its greater lightness and capacity to resist corrosion, and the possibility that they are manufactured by deep stretching in complex shapes.

However, the rules currently in force increasingly tend to reduce the amount of gaseous emissions and of fuel vapors from the tanks, in order to reduce environmental pollution. For this purpose, it must be minimize gas and vapor emission paths through of the openings in the walls of the fuel tanks, necessary for connection to accessories and parts components. Additionally, the current approach is to install a large part of the components and accessories necessary for the feeding the fuel to an engine or user within its own fuel tank.

In the manufacture of deposits plastic fuel, use is usually made of the blow molding technology, according to which it is extruded a preform or a tubular element of plastic material between two semi-molds, which are subsequently held to proceed to the peripheral clamping of the preform deposited between them; a pressurized fluid is then injected into the preform to cause dilation and adhesion to the internal surfaces of the mold.

In the manufacture of deposits plastic storage, in order to provide resistance structural necessary, and the tightness or barrier required to hydrocarbon gases, a material is generally used layered plastic overlapping a certain number of plastic sheets that They have different chemical and / or physical properties.

In the manufacture of deposits fuel by blow molding technology, after the conformation stage, it is necessary to make some openings in the tank walls to install the various components, both inside as outside the deposit. All this implies some solutions extremely complex, long manufacturing procedures and in somewhat expensive, as well as a high risk of issuing hydrocarbons through the openings in the deposits of fuel, in case they don't close tightly with precision. In addition, whenever it is necessary to manufacture the tanks Fuel in multilayer material, any control of the wall thickness proves to be extremely difficult to achieve.

In order to partially obviate these disadvantages and to achieve high production in quantitative terms, the US 6,372,176 and WO 02/14050 propose the use of the known thermal sheet forming technology twins

According to this technology, the material sheets thermoplastic are heated and subjected to process in a station of respective molding in which each individual sheet of material is thermally formed in a housing, inside a mold correspondingly, two housings are subsequently joined and tightly close around its peripheral edges to Form a fuel tank.

According to these documents, the first one is heated and the second plastic sheets and undergo process along some independent process lines in which each of the plastic sheets travel from a loading station, to a thermal forming station in which it is thermally formed each preheated sheet in a respective female mold; a first These mold is mounted in an upward-facing arrangement in a lower plate, while the second mold of these is ride in a downward-facing arrangement on a plate Superior of a press.

After the two sheets have been thermally shaped, the upper mold must be aligned first with respect to the lower mold and then it lowers to apply a force to melt and seal the thermally shaped housings along its edges peripherals

WO 03/097330 also refers to an apparatus for the thermal forming of hollow articles of twin sheet plastic in which some use is made again thermal forming molds facing up and towards down.

Operators can enter various inserts and / or components in preset positions before to join and seal the two housings to form a Deposit.

In fact, according to the technical documents Previously mentioned above, the thermal conformation of the Lower housing is performed while maintaining an oriented arrangement upward from the lower mold cavity while the Thermal conformation of the upper housing is done by maintaining an arrangement facing down the mold cavity higher.

All this brings difficulties considerable for the thermal conformation of the upper housing, as well as a lack of structural uniformity in the deposit of fuel, due to a bending and a different stretch of the plastic sheets, in particular, of the upper face, because at be oriented the cavity of the upper mold down, the bending is the opposite of the bending caused during the heating of the plastic sheet

Document US 3 779 687 shows a plant for the thermal conformation of twin sheets where both molds They are facing up. Also, the conditions of different stretch of plastic sheets caused by the combined cause structural inequalities and differences in thickness in the housings, which are difficult to remove.

In addition, the different arrangement of the molds in the two process lines complicates the insertion of components in the fuel tank, doing the installation extremely laborious and difficult for an operator to obtain the access in order to perform the necessary verifications and inspections The replacement of molds and elements or Templates is also difficult to perform.

Therefore, in the manufacture of plastic fuel tanks according to current technologies large amounts of cuts are produced, and since certain expensive plastic materials are difficult to recover, there is a big problem to find new molding systems in the which, in addition to improving production, can be reduced to minimum cuts and, consequently, the molding costs of deposits

Although the solutions proposed in the prior art documents allow the use of conformation Twin sheet thermal to obtain high volumes of production, is susceptible to additional improvements aimed at improve both the manufacturing process and the deposits of thermally formed fuel.

Objects of the invention

Therefore, the main object of this invention is to provide a method and a plant for thermal conformation of hollow bodies, in particular fuel tanks, by means of twin-sheet thermal forming technology, capable to provide further simplification of the manufacturing cycle, and of processing the sheets of substantially identical plastic designed to form the two housings of a tank fuel.

Another object of the invention is to provide a method and a plant, as mentioned above, by means of which is possible to manufacture characterized fuel tanks by a high degree of structural homogeneity.

A further object of this invention is provide a method and plant for the manufacture of deposits  of fuel through conformation technology Twin-sheet thermal that, in addition to working simultaneously on both process lines, make it equally possible work with extremely short production cycles so substantially continuous, without stopping tempos, simplifying considerably the insertion of the components in the molds

Another object of this invention is to provide a method for thermal conformation of fuel tanks of plastic material with which it is possible to substantially reduce the problems related to recovering the cuts, because It allows less loss of valuable material.

Another additional object of the invention is provide easy accessibility to the shaping area as well how to facilitate maintenance operations for all plant.

Brief Description of the Invention

According to the invention, everything can be achieved above by means of a method for thermal conformation in twin sheets of fuel tanks according to claim 1, and with a plant according to claim 7.

Before heating to the temperature of thermal conformation, each plastic sheet can be subjected to a preheating followed by centering at a centering station successive to allow a correct pneumatic grip of the edges leaf peripherals; if required, centering can precede The preheating stage. Additionally, using some special pneumatic racks, it is possible to substantially reduce the grip surfaces of the sheets and therefore the material cuts.

According to an additional feature of the invention, the substantially flat state of the plastic sheets,  both during heating and leaf movement heated towards the thermal forming station, it can be properly controlled by changing the softening temperature of the plastic material and / or adjusting the vacuum inside the device of support.

For the purposes of this specification, the term "substantially flat" is understood as meaning of a condition in which the heated sheet is held pneumatically at the peripheral edges, and is vacuum supported on the upper side, without being combated down by one degree important, due to gravity.

According to another feature of the invention, the stages of heating, centering and thermal conformation of plastic sheets along the process lines can be perform cyclically, feeding the individual sheets at along the two process lines while several operations simultaneously or in succession.

According to the invention, after forming thermal and sealing of the two housings in a tank, it is possible perform a cooling stage of the fuel tank in a separate cooling station, arranged next to the process lines Since the cooling of the molds and the thermally formed tank occupies a time interval considerable, in this way it is possible to cool the deposits after of thermal conformation, without affecting the manufacturing process Or stop it. This can be achieved by providing, on one side of a of the two process lines, in correspondence with the station of thermal conformation, a cooling station comprising a rotating table or an alternative shuttle that has two or more mold support areas, which can be aligned once in a while in time with the thermal forming stations of the lines process, to support the closed molds and the tank thermally shaped.

The two molds closed with the tank can be simply transferred on the turntable or on the support shuttle Therefore, provided that the sealing of the two housings and the storage tank cooling they take place by feeding a pressurized fluid in it deposit, in correspondence with the cooling station, is it is necessary to make use of a suitable clamping press to maintain   the tank closed in the two molds. Optionally it is possible make use of a special box to contain the closed molds, as an alternative to the press in the cooling station.

According to an additional feature of the invention, a method and a plant for the thermal conformation of storage tanks, by means of which can be arranged and placed in parallel the two lines of process with both molds facing up on the same level; this greatly facilitates the access of one or more operators to the entire plant, for the necessary controls, such as for the introduction of the components in the two housings, as well as for all necessary maintenance operations. It also greatly facilitates the replacement of molds and It can be done outside the two process lines, in correspondence with the cooling station.

Brief description of the figures

They will be more clearly evident these and other advantages of the invention and the characteristics of the method and of the plant according to this invention from the description next, which refers to the attached drawings, in the which:

Fig. 1 shows a block diagram of the various stages and workstations according to one embodiment preferred of the invention;

Figs. 2A-2H show about flow charts illustrating the various operations performed along each process line;

Figs. 3 and 4 show a view from above and a side view respectively of a book press of a thermal forming station, in an open condition;

Fig. 5 shows a view of a bell of vacuum to pneumatically hold and vacuum support the plastic sheets along each process line;

Fig. 6 shows an enlarged detail of Fig. 5;

Fig. 7 shows a top view of the plant, in correspondence with the thermal forming station, according to a first embodiment;

Fig. 8 shows a view similar to that of the preceding figure, according to a further embodiment;

Figs. 9 and 10 show a front view and a side view, in open and closed conditions, of a box to hold the molds closed during cooling.

Detailed description of the invention

As shown in Fig. 1, the conformation of twin-leaf fuel tanks take place processing individual sheets of plastic material that can be shaped thermally along two separate process lines 10A and 10B, which extend parallel from a station of load for plastic sheets, to a forming station thermal through intermediate work stations; to what along the two process lines 10A, 10B, the plastic sheets individual are heated and shaped in the respective thermally shaped housings, subjecting the plastic sheets at the same process stages, while moving in pairs through the various work stations of a plant.

For the purposes of this description, it is understood that "thermally formed plastic sheet" means any sheet-shaped plastic material, suitable to be given a form by a thermal forming process; You can also arrange the assistance of a male. The sheets of plastic can be well of a single layer of a suitable thickness, or multilayered, that is, composed of several layers of a plastic material of identical or different thickness, which have chemical and / or physical properties that differ from each other.

As shown in Figure 1, each line of process 10A, 10B comprises a number of workstations in which are carried out the various stages of the process; in in particular, in a first station 11A, 11B, a first loading stage capturing the individual sheets SA, SB of a pallet

The individual plastic sheets SA, SB are Automatically captured and fed to charging stations 11A, 11B by means of mechanical gripping devices and / or tire; subsequently they are transferred to a station preheating 12A, 12B in which the sheets of plastic in a heater for the time necessary to bring them to a first preheat temperature lower than the thermal forming temperature of the same sheets. Yet when it is preferable to preheat the plastic sheets SA, SB in order of reducing work cycle times, you can also skip the preheat phase.

The preheating temperature for plastic sheets SA, SB is adequately controlled, for example causing the leaves to move along a path of heating inside a convection air heater, in the which leaves remain during a warm-up period equivalent to several process cycles, during which the leaves They are gradually heated to a desired temperature.

At the end of the preheating stage, each Individual sheet SA, SB is transferred to a centering station 13A, 13B subsequent, where the sheet is placed and oriented so that be pneumatically captured, as explained below.

In the case of figure 1, the centering stage 13A, 13B is done downstream, immediately after preheating; in certain cases, this would imply having to maintain a relatively low preheat temperature with with respect to the temperature of thermal conformation, increasing of This way the final warm-up time. In order to get some better centering conditions for the leaves, the latter they can be centered preliminary, prior to their introduction in the preheating oven; this realization offers the possibility of increasing the preheating temperature and therefore reduce the final heating time.

According to the diagram as an example of the Figure 1, after the preheated sheets have been centered at stations 13A, 13B, the individual sheets are transferred to a subsequent final heating station 14A, 14B in which they are heated to a temperature close to or equivalent to a thermal forming temperature, depending on the properties of the plastic material or materials of which each individual sheet SA, SB.

In relation to this, as indicated schematically in figure 1, the individual sheets SA, SB in centering stations 13A, 13B are captured by means of a suitable pneumatic grip device 15A and 15B, movable above each of the process lines, for example the type shown in figures 5 and 6.

Figure 5 shows, by way of example, a cross-sectional view of the capture device 15A and pneumatically operated vacuum retention; the pneumatic device Capture 15B is completely similar to device 15A.

As shown, the capture device 15A pneumatic and vacuum retention is equipped with a device in bell shape that has peripheral walls 21 that form a vacuum chamber 22 facing down; the vacuum chamber 22 It is equipped with a conical top that ends with a accessory 23 for connection to an air intake source 23 ', necessary to create a sufficient degree of vacuum in the chamber vacuum 22 to support or retain the sheet of plastic material SA in a substantially flat condition; controlling the source of suction 23 'it is possible to control the degree of vacuum in the chamber vacuum 22 of the bell-shaped device 15A and consequently the flat condition of the SA plastic sheet.

The shape and size of the device 15A correspond substantially to those of SA, SB plastic sheets to be thermally shaped. Therefore, the device in bell shape 15A of figures 5 and 6 is arranged at the edge bottom, with a grip that can be operated pneumatically to grab the SA sheet around its edge peripheral, forming a suitable air tight seal.

As shown in figure 5, and in detail enlarged from figure 6, the pneumatic gripping means for grab the SA sheet comprise a flat peripheral frame 24 fixed within the side walls 21 and separated from them of so that a slot 25 is formed that can be connected in one controlled way to a suction source 25 '.

Pneumatic gripping means to grab the plastic sheets can obviously be of shapes or conformations different, compared to those shown. Finally, also in the Figure 5, it can be seen that the bell-shaped device 15A to grab and vacuum hold the sheets SA, SB, is equipped with a battery 28 of heating elements to heat the sheets SA, SB, within the same vacuum chamber 22.

Capture and move each individual SA, SB sheet preheated constitutes one of the most delicate stages of the entire process, because, if the preheated sheets were not properly captured and supported, due to gravity they would tend to fight down, and would experience uncontrolled deformation, the which would have a negative effect on the subsequent conformation thermal of the housings in stations 16A, 16B.

Therefore, according to a preferred embodiment of the invention, the individual SA, SB sheets are pneumatically captured at the respective centering stations 13A, 13B in a manner controlled at its peripheral edges, and are supported by vacuum by means of the bell-shaped devices 15A, 15B of the Figure 5, keeping them in a substantially flat condition while they are transferred to heating stations 14A, 14B. In these seasons, the leaves are heated further individual SA, SB to bring them to an equivalent temperature or next to the thermal forming temperature. The warm-up of the SA and SB sheets in the two heating stations 14A and 14B can take place on both sides from below by means of a battery of heating elements 47 of which each station 14A, 14B, and from above by means of the battery of heating elements 28 inside the device 15A, 15B in shape of bell.

After the stage of heating to the thermal forming temperature in the 16A, 16B stations, heated SA, SB plastic sheets are transferred by means of devices 15A, 15B at subsequent stations 16A, 16B to be subjected to the thermal conformation in molds 17A, 17B to give them the shape corresponding to GA, GB thermally shaped housings according to the procedure further illustrated in the figures 2A-2H of the attached drawings.

As previously mentioned, the transfer of the heated sheets SA, SB takes place while they are supported  by vacuum in a flat or substantially flat condition, that is, free from deep combing, by means of devices 15A, 15B bell-shaped, which can be displaced by both quickly between the various workstations of the line.

In order to maintain a condition substantially flattened heated sheets, preventing the latter from beat excessively as the temperature of heating, the combustion of the leaves is detected and controlled constantly the degree of vacuum created above the sheets heated by devices 15A, 15B and adjusted so that excessive bending is prevented with the risk that the leaves of plastic knock against the fixed parts of the plant, preventing or hindering its correct placement on top of the 17A molds, 17B.

As shown in more detail below, The essential feature of this invention lies in the thermal conformation of SA, SB plastic sheets while maintains thermal conformation conditions for both substantially identical, so that you get two housings thermally formed GA, structurally homogeneous GB.

According to the invention, this can be obtained. arranging both molds 17A, 17B side by side at the same level, with the respective conformation cavities oriented towards above.

The side-by-side and facing orientation above the two thermal forming molds allows the sheets are also stretched previously and deposited on the molds simply by gravity. This not only simplifies the thermal conformation greatly, making it possible to obtain the Desired structural homogeneity of the two GA, GB housings, but which also facilitates the operations of inserting inserts and components in one or both housings formed before sealing the Deposit. In fact, thanks to the upward-facing arrangement of the cavities for both molds 17A, 17B, it is possible to perform introduction of the inserts and / or the various components, both before and after thermal conformation, directly at each mold or in thermally shaped housings, while these last remain still in the respective mold, capturing by an operator the various components of a side station 18A and 18B, as shown in greater detail in Figures 7 and 8.

At the end of the thermal conformation of the two housings and the introduction of the various components, as previously mentioned, it can still be done in one of the thermal forming stations a subsequent stage consisting of placing the two molds 17A, 17B one on top of the other melting and sealing the two thermally shaped housings in overlapping sealing areas, as shown schematically in the block diagram 19 of figure 1.

This can be achieved in any suitable way, for example, by turning one of the molds upside down on the other, by simple rotation on a horizontal axis, or in any other way capable of allowing
the superimposition of the two molds with the respective thermally shaped housings, one of them
turned upside down on top of each other and with their respective peripheral sealing areas coincide-
tes.

In this regard, as shown schematically in figures 3 and 4, use of a Book type press.

The thermal forming press comprises a stationary frame 30 and a movable frame 31, which they can be rotated 180º around an axis of rotation 32 connected to a drive motor 33, or in any other way adequate.

The stationary frame 30 of the press is provided in turn with a plate 34 to support the mold 17B; he plate 34 can be moved vertically by means of cylinders hydraulic 35 in order to provide the clamping force necessary to close the molds 17A, 17B and seal the areas overlapping of the two housings, in the closed condition of the molds shown by the block diagram 19 of Figure 1.

It can also be seen in figures 3 and 4 that, according to another feature of the invention, means are provided of grip of the leaves that can be operated pneumatically to each mold 17A, 17B; the gripping means comprise a frame tire 36A, 36B to grip the SA, SB blades from below, around its peripheral edges, on the opposite side of the grip device 24 of the bell-shaped device 15A, 15B as explained below. In this sense, the pneumatic racks 36A, 36B are equipped with a shape and a size identical to those of bell-shaped devices 15A and 15B. This proves to be extremely advantageous because it allows the individual sheets are supported along the two process lines 10A, 10B and be transferred by the devices bell-shaped 15A, 15B above the molds of conformation 17A, 17B, grabbing the leaves themselves along a narrow peripheral band. This makes it possible to reduce so considerable cuts and losses of valuable material, in comparison with conventional transport systems used previously in conventional thermal forming plants which also make use of belts or other transport systems Similar for plastic sheets.

Each pneumatic frame 36A, 36B can be vertically displaced upwards, with respect to the molds 17A, 17B, by means of suitable control cylinders 37A, 37B. The 36A and 36B pneumatic grip frames may have a flat or variable profile shape, providing sections of articulated frame, to fit molds with shaped edges flat or three-dimensional; In addition, each frame can be provided also of mechanical gripping means to hold the edges of the individual sheets.

Once the two molds 17A, 17B have been placed with the corresponding thermally shaped housings a on top of the other, the corresponding areas are pressed sealing peripherals of the two housings and merge the one with the other by the sealing areas of the edges of the molds themselves of thermal conformation, melting and sealing of This forms a fuel tank.

Once the thermal conformation and the sealing of the two housings, it is necessary to perform a stage of cooling to cool the tank before removing it from baking tin.

The cooling can be done from any properly, for example by circulating water or a fluid of cooling within the walls of the mold, or by circulation of air while maintaining the tank in a condition of pressurization to push it against the molds of thermal conformation

Although cooling can be performed directly along the process line, in one or both thermal forming stations 16A, 16B, since the cooling of molds and fuel tanks takes a considerable time interval, according to another characteristic of this invention is preferable to perform cooling separately of the process line; in this sense, the molds closed with the  Thermally formed deposit are transferred to a station cooling 20 to one side of the process lines 10A, 10B. Removing the closed molds and cooling in a side station, separated from the process lines, is possible operate in a continuous cycle, without causing delays or downtime in the manufacturing process, which can continue as explain later. Additionally, removing the molds from the process lines and transferring them to a cooling station separate, it is possible to easily work in the molds themselves to maintenance operations and / or for replacement.

After melting and sealing of the two housings, depending on whether the cooling is done with the fuel tank under pressure conditions or no, the closed molds of one of the stations must be transferred of thermal conformation to the cooling station.

In case the cooling is done with the tank under pressure conditions, in order to counteract the pressure inside the molds, it is advisable to close the latter in a special retention box, shown in the Figures 8 and 10 of the attached drawings.

For example, as shown in Figures 9 and 10, the box may comprise a lower plate 40 and a plate upper 41 in which the two molds 17A, 17B are held. He upper plate 41 is articulated to two side posts 42 for rotate around an articulation shaft 43 capable of sliding vertically along an elongated hole 42 '. The movement rotational upper plate 41 is controlled by two cylinders hydraulic 44, while wedges 45 driven by Hydraulic cylinders 46 allow the top plate 41 to be interlocked by the two side posts 42, in the closed condition of figure 9.

Referring to the diagram of the figures 2A-2H, a further description is given below detailed method of operation of the forming plant Twin sheet thermal according to this invention.

The figures from 2A to 2H show schematically the individual operational stages that are perform in parallel and in succession in the various stations of work along the two process lines, for example between the centering station 13A of line 10A, and the station thermal conformation 16A, it being clearly understood that simultaneously perform the same process steps in parallel in the other process line 10B.

As initially mentioned, the sheets of Individual plastic SA of charging station 11A, are captured  and introduced into the oven 12A where they are retained during a preset period of time, on the order of a few dozen of minutes, depending on the nature and properties of the plastic material, making them experience a preheat gradual at a lower first temperature than conformation thermal

After finishing the preheating phase, at the exit of the oven 12A the preheated sheet SA is transferred to the centering station 13A, for example by means of a table of rollers 45 or other transport system in which a appropriate centering device 46 (Figure 2A) for positioning and accurately orient each sheet SA1, so that it results perfectly aligned with the pneumatic grip device 15A, which has meanwhile moved above the station of centering 13A.

Once the centering of sheet SA1 is completed, the latter is pneumatically grabbed around its edge peripheral and lifted by means of the pneumatic grip device 15A, which is supported to move vertically and / or horizontally in the directions of the double arrows F2 and F3, above the process line 10A.

The 15A device for gripping the sheets has one way and acts to pneumatically capture the SA1 sheet and form a seal around the peripheral edge so that above the leaf itself inside the bell 15A is possible to create, so controlled, a certain degree of vacuum capable of retaining the SA1 sheet in a substantially flat condition, as shown.

Accordingly, the gripping device 15A with sheet SA1 you can move from centering station 13A to the heating station 14A and the forming station thermal 15A, as schematically shown in Figures 2A, 2B, 2C and 2D of the attached drawings.

In correspondence with the centering station 13A, the pneumatic grip device 15A captures the sheet SA1 already centered, keeping it pneumatically around the edge peripheral.

Immediately after, the heater 28, the bell 15A is connected to a vacuum source and to then, from centering station 13A the 15A pneumatic grip device with preheated SA1 blade towards the final heating station 14A as shown in the figure 2C.

During this stage, the sheet material of plastic SA1, analogous to sheet SA2, is heated in addition to the required thermal forming temperature, both by the upper heater 28 of the gripping device 15A as by a lower heater 47 placed underneath itself heating station 14A. Since during this stage of heating the plastic material of the sheet is taken to a temperature close to its melting point, and therefore would tend to beat down by gravity, the vacuum is maintained in the hood of the gripping device 15A and it is controlled properly to keep sheet SA1 in a condition substantially flat, throughout the entire period of heating until it has been placed on top of the mold thermal conformation 17A.

Meanwhile, several operators have been busy of inserting the inserts into the mold 17A, as shown schematically in figure 2C.

The plastic sheet SA1 is then heated on both sides in a totally controlled way. At the end of heating, the grip device 15A is displaced quickly above the thermal forming mold 17A, as It is shown in Figure 2D.

The grip and retention device 15A by vacuum with SA1 sheet heated to forming temperature  thermal, is transferred while maintaining heating by heater medium 28 and vacuum for the reasons above explained.

At this point, reducing the vacuum, or controlling the pressure inside the gripping device 15 and retention, it is possible to form a gravity batter, causing the sheet is combated down, as shown in Figure 2E; simultaneously, the second gripping device 36A, associated with the 17A mold, rises to grab the SA1 sheet from below, around of the peripheral edge, on the opposite side of the grip frame upper 24 of the grip and retention device 15A, as shown again in figure 2E.

Meanwhile, a new one can be transferred preheated sheet SA2 to centering station 13A.

At this point, device 15A releases the blade SA 1 which is immediately grabbed from below by the second gripping device 36A of the thermal forming mold 17A; be can now return to the first gripping device and retention 15A towards centering station 13A in which it can capture a second sheet SA2, as shown in Figure 2F.

The same figure 2F shows that it is lowered to second gripping device 36A that retains the first sheet SA1 in a combated condition, and stops at a distance from the mold 17A, while a shaping male 49 is made to adhere from above to the sanding of sheet SA1.

Then the male is lowered conformation 49 as the grip device 36A simultaneously taking sheet SA1 to rest against the inner surface of the 17A mold. Activating the vacuum in the mold 17A, in a way known in itself, the sheet SA1 is shaped in a housing thermally shaped by the combined action of the vacuum inside of the mold 17A and the pressure of the forming male 49.

During the thermal conformation of the housing, the peripheral edge of the plastic sheet SA1 is retained against the peripheral edge of the mold 17B, for example by means of a clamp 48, or by any other suitable way, as shown in the figure 2G.

At the end of the thermal conformation of the housing, when the plastic material is still at a temperature high, the shaping male 49 is raised to allow introduction of any other inserts or components in the thermally shaped housing. You can do all this fast and easily through the upward orientation of the molds, which simplifies all operations in both process lines

As previously mentioned, all operations described with reference to the figures from 2A to 2H They are performed simultaneously and cyclically on two plastic sheets SA1, SB1 along the two process lines 10A and 10B. By consequently, in both cases, the thermal conformation of the two carcasses takes place in a substantially identical way, with both molds facing up; In addition, in both cases, properly use emptiness and gravity to control the formation of the blanket in each of the sheets SA1, SB1.

Now it is necessary to seal the two housings to Form a fuel tank. In this sense, the Hermetic sealing of the two thermally shaped housings squeezing tightly and melting its sealing areas the one against each other around the peripheral edges of the two housings

You can achieve this, for example, by giving the 180º turn to one of the molds, for example making the mold 17B rotate in the direction of arrow F1 of figure 4, around a rotational axis 32, standing upside down from the other mold 16A, as shown in the block diagram 19 of the figure one.

At this point, the two molds are tightened with force against each other, exerting a fixing force adequate, and pinching the two sealing areas of the two housings melting them in this way so that they remain tightly together.

After completing a deposit of fuel, the latter can be cooled while maintaining Closed in both molds. At the end of the cooling, you can move the two still closed molds back to one of the two thermal forming stations 17A, 17B and open them for perform the extraction of the mold and the discharge of the tank finished fuel, directly at the same station thermal conformation

You can do all this within a period of extremely short time, during the execution of a cycle subsequent operation, so that the same molds are used again for the manufacture of an upcoming warehouse of fuel.

Therefore, working in a continuous cycle with the two process lines 10A and 10B, according to the procedures previously described, it is possible to produce fuel tanks plastic, complete with the respective accessories and components in an extremely limited period of time, reducing material cuts due to the combined use of the two grip devices and pneumatic transport along the two process lines, it becomes possible to form a seal that has some extremely limited surfaces; according to the invention, it is It is also possible to produce complete plastic tanks with their respective accessories that have structural characteristics improved.

Figures 7 and 8 show, by way of example, two possible solutions with respect to the cooling station, in addition to other details of the forming stations thermal and stations to introduce the inserts and / or others components inside the molds and / or inside the housings thermally shaped; in said figures, the same are used reference numbers than in the previous figures to indicate parts  similar or equivalent.

As shown in Figure 7, the station of cooling 20 comprises a rotary table 50 that has two or more support surfaces 51, 52 on which the closed molds 17A, 17B for the cooling phase.

The rotary table 50 is made to rotate and is indexed by a motor 53 in order to occasionally align a of the support surfaces 51, 52 of the molds with the station of thermal conformation, to allow the transfer of closed molds, for example, between the forming station thermal 16B and surface 51, while another closed mold that experience cooling is already arranged on surface 52 or on the other surfaces of the rotary table. Figure 7 indicates schematically several robots or operators for the movement of the shaping male 49, or to capture and introduce automatically the inserts in the molds, or to download the finished fuel tanks.

The example in figure 8 differs from the figure precedent in which the cooling station 20 now comprises a shuttle 54 with an alternative movement along a guide 55, which extends on a side parallel to the line of process 16B. The shuttle 54 is provided with two surfaces of mold holder 56, 57, which can be aligned with the thermal forming station 16B, properly displacing the shuttle along guide 55.

In both cases, surfaces can be used 51, 52 of the turntable 50, or the support surfaces of mold 56, 57 of shuttle 55 to carry out the replacement of the molds, at each production change, or for others requirements

It is understood that what is described with reference to annexed drawings have been given purely by way of example in order to illustrate the general characteristics of the method and the plant according to this invention; therefore, you can perform other modifications or variations, without departing from the claims.

Claims (20)

1. Method for thermal conformation of plastic fuel tanks, according to which a first and a second sheet (SA, SB) of plastic material that can be thermally shaped are independently heated and displaced along a first (A) and respectively along a second (B) process line from a station load (11A, 11B) to a respective thermal forming station (16A, 16B), the method comprising the main stages of:

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subject each plastic sheet (SA, SB) to a warm up;

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pneumatically hold the sheets of heated plastic (SA, SB) along its peripheral edges by air aspiration, and vacuum support the same sheets (SA, SB) in a substantially flat condition by controlling the degree of vacuum while moving along the respective process lines (A, B);

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place each heated plastic sheet (SA, SB) above a forming mold (17A, 17B) respective that has an upwardly oriented cavity while the sheet (SA, SB) is still pneumatically held in the substantially flat condition mentioned above;

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do lower the heated plastic sheet (SA, SB) into a mold (17A, 17B) respective; Y

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thermally shape each sheet of heated plastic (SA, SB) in a respective housing (GA, GB), melting the same sheet (SA, SB) to adhere it to the cavity open upward of the forming mold (17A, 17B);

the method also comprising the steps supplementary to:

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give the turn from top to bottom (17B) of the forming molds (17A, 17B) and to the thermally shaped housing (GB);

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put on said mold (17B) that has been turned over the other mold (17A) facing up, so that the areas overlap sealing peripherals of the two housings (GA, GB) formed thermally placed on top of each other; Y

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melt and heat weld the overlapping sealing areas of the housings (GA, GB) by tightening said overlapping sealing areas between clamping surfaces of forming molds (17A, 17B).

2. Method for thermal sheet forming twin fuel tanks according to claim 1, comprising the steps of preheating (12A, 12B) each sheet of plastic (SA, SB) at a first preheat temperature lower than a thermal forming temperature, and keep the blade heating (SA, SB) while traveling along of the process line (A, B). 3. Method for thermal sheet forming twin fuel tanks according to the claims 1 and 2, which includes the steps of controlling and adjusting the degree of vacuum to support the sheet (SA, SB) in order to prevent warmed during heating. 4. Method for thermal sheet forming twin fuel tanks according to claim 1, comprising the steps of introducing the inserts and / or components for the fuel tank in the cavity facing above the molds, before the thermal conformation of the sheets of plastic (SA, SB). 5. Method for thermal sheet forming twin fuel tanks according to claim 1, comprising the steps of introducing the inserts and / or components of the fuel tank in the housings (GA, GB) formed thermally through the cavity facing upwards of the molds (17A, 17B). 6. Method for thermal sheet forming twin fuel tanks according to claim 1, comprising the steps of removing the closed molds (17A, 17B) and of cooling the same closed molds (17A, 17B) outside the process lines. 7. Plant for manufacturing fuel tanks of plastic comprising a first and a second housing (GA, GB) thermally shaped in twin sheets, in which a first and second sheets (SA, SB) of plastic that can be thermally shaped are independently heated and shifted along a first and second lines of respective process (A, B), from a charging station (11A, 11B) to through at least one heating station (12A, 14A; 12B, 14B), towards a respective thermal forming station (16A, 16B), where the individual plastic sheets (SA, SB) are thermally formed in a first and a second housing (GA, GB) within a first and second forming mold (17A, 17B) respective, where

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sayings first and second forming mold (17A, 17B) are arranged side by side with the open cavities of both molds (17A, 17B) facing up;

where each process line (A, B) comprises racks (24A, 24B) with air intake to grab the plastic sheets (SA, SB) along their peripheral edges, and a vacuum sheet retention device (15A, 15B) that it comprises a vacuum chamber (22) to retain the sheets of plastic (SA, SB) heated, being able to move said racks (24A, 24B) with air intake and said device vacuum retention (15A, 15B) along the process lines (A, B); and vacuum control means comprising a source Adjustable vacuum to control the degree of vacuum in the chamber vacuum (22) of the vacuum retention device (15A, 15B), for support the plastic sheets (SA, SB) heated in a substantially flat condition; Y drive means (33) formed and arranged to turn a mold upside down (17A, 17B) to place it on top of the other (17A, 17B) and cause welding of overlapping sealing areas of shaped housings thermally (GA, GB) by compression of the sealing areas overlapped by the same molds (17A, 17B). 8. Plant for manufacturing fuel tanks plastic according to claim 7, comprising a station preheating (12A, 12B). 9. Plant for manufacturing fuel tanks plastic according to claim 7, comprising a station of centering (13A, 13B) of leaves. 10. Plant for manufacturing deposits of plastic fuel according to claims 8 and 9, wherein the centering station (13A, 13B) of leaves is arranged waters above the preheating station (12A, 12B) of sheets. 11. Plant for manufacturing deposits of plastic fuel according to claims 8 and 9, wherein the centering station (13A, 13B) of sheets is arranged between the preheating station (12A, 12B) and a second station of heating (14A, 14B) for plastic sheets (SA, SB). 12. Plant for manufacturing deposits of plastic fuel according to claim 7, wherein the support device (15A, 15B) by vacuum comprises about heating elements (28) for plastic sheets (SA, SB). 13. Plant for manufacturing deposits of plastic fuel according to claim 7, wherein each mold (17A, 17B) comprises a second gripping means (36A, 36B) of blades that can be operated pneumatically. 14. Plant for manufacturing deposits of plastic fuel according to claim 7, wherein said vacuum retention device (15A, 15B) has the form of a suction hood that can be operated pneumatically. 15. Plant for manufacturing deposits of plastic fuel according to claim 7, comprising a  cooling station (20) of molds next to the lines of process (A, B), in a laterally aligned condition with a thermal forming station (16A, 16B), to transfer the closed molds (17A, 17B) between the forming station thermal (16A, 16B) and the cooling station (20) of the plant. 16. Plant for manufacturing deposits of plastic fuel according to claim 15, wherein the cooling station (20) comprises a rotary table (50) that has a plurality of support surfaces (51, 52) of molds 17. Plant for manufacturing deposits of plastic fuel according to claim 15, wherein the cooling station (20) comprises a shuttle with alternative movement (54) of mold support arranged parallel to the process lines (A, B), said provision being provided shuttle (54) of at least one first and second surfaces (55, 56) mold support. 18. Plant for manufacturing deposits of plastic fuel according to claim 15, wherein said means for transferring the molds (17A, 17B) comprise a box of clamping (40, 41) of molds that moves alternately between a thermal forming station (16A, 16B) and the station cooling (20). 19. Plant for manufacturing deposits of plastic fuel according to claim 7, wherein said drive means for turning upside down to a Mold (17B) comprise a book press. 20. Plant for manufacturing deposits of plastic fuel according to claim 13, wherein each mold  (17A, 17B) comprises additional mechanical means (48) for grab the edges of the heated plastic sheets (SA, SB).